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The influence of methyl substitution on the thermodynamic parameters for the binding of [Fe(DMP) 3 ] 2+  and [Fe(TMP) 3 ] 2+  (DMP = 4,7-dimethyl-1,10-phenanthroline, TMP = 3,4,7,8-tetramethyl- 1,10-phenanthroline) to calf thymus DNA (ct-DNA) has been studied by determining their equilibrium binding constants (K b ) at various salt concentrations and temperatures. K b  of the iron(II) complexes to ct-DNA decreases with the salt concentration in the solution, suggesting considerable electrostatic interaction in the ct-DNA binding of the iron(II) complexes. In contrast, K b  of the DNA binding increases with temperature, indicating that the DNA binding reaction of the complex is endothermic and entropically driven. The evaluation of the non-electrostatic binding constant (K 0  t ) based on polyelectrolyte theory has revealed that the K 0  t  portions of the total binding constant (K b ) are relatively large and reach 46.4% for [Fe(DMP) 3 ] 2+  at [Na + ] = 0.075 M and 43.9% for [Fe(TMP) 3 ] 2+  at [Na + ] = 0.100 M. The contribution of non-electrostatic binding free energy (ΔG 0  t  ) to total binding free energy change (ΔG 0 ) is extremely large, i. e. &gt; 90% for both iron(II) complexes at [Na + ] = 0.05 M, suggesting that the stabilization of the DNA binding is mainly contributed from the non-electrostatic process. The effect of methyl substitution on electrostatic (ΔG 0  pe ) and non-electrostatic (ΔG 0  t  ) binding free energy changes has been systematically evaluated using the quantity of ΔΔG 0  pe  and ΔΔG 0  t  relative to that of the parent iron(II) complex, [Fe(phen) 3 ] 2+ . The results indicate that the substitution of hydrogen atoms in the phen ligand by methyl groups decreases slightly the electrostatic binding free energy changes, but tremendously increases the non-electrostatic ones to yield net binding free energy changes which are more favorable for the ct-DNA binding.

DNA Binding of Iron(II)-Phenanthroline Complexes: Effect of Methyl Substitution on Thermodynamic Parameters